Molded sound reproducer and method of making the same



June 5, 1956 H. o. FUCHS 2,749,393

MOLDED SOUND REPRODUCER AND METHOD OF MAKING THE SAME Filed May 1, 1951IN V EN TOR.

BMW/6m;

United States Patent MOLDED SOUND REPRODUCER AND METHOD OF MAKING THESAME Harold 0. Fuchs, Carle Place, N. Y., assignor to PickeringAssociates, Inc., Oceanside, N. Y., a corporation of New YorkApplication May 1, 1951, Serial No. 224,029

6 Claims. (Cl. 179100.41)

This invention relates to a molded construction for a 2,749,393 CePatented June 5,1956

' netic material fixedly and rigidly mounted near the free end of achannel-shaped cantilever spring 17 and projecting downwardly throughthe gap between the ends of the pole pieces 11 and through the center ofthe spool 14 and the coil 15. The cantilever spring 17 is secured'to thecenter of the bar magnet 12 which in turn is a part of the soundreproducer for phonographs and more particularly to a sound reproducercast in a resin.

It is an object of this invention to provide a sound reproducer,particularly a magnetic sound reproducer, in which all of the parts arecast in a solid mass of resin having only a stylus and contact prongsprotruding.

It is a further object of this invention to provide a method of assemblyof a magnetic sound reproducer and easing the same in a solid body ofresin which is easy and inexpensive.

These and other objects of this invention will become more apparent uponconsideration of the following description taken together with theaccompanying drawings in which:

Fig. 1 is a plan view of the magnetic sound reproducer of the type whichmay be used in this invention, positioned in a mold;

Fig. 2 is a longitudinal, vertical section of the magnetic soundreproducer of this invention in a mold;

Fig. 3 is a vertical, transverse section of the armature in the moldtaken on line 33 of Fig. 2;

Fig. 4 is a longitudinal, vertical section of the finished magneticsound reproducer of this invention, and

Fig. 5 is a bottom perspective view of the magnetic sound reproducer ofthis invention.

This invention provides a sound reproducer assembly cast within a solidmass and incorporating a core of viscous fluid material which permitsreciprocation of the pickup armature and at the same time dampens thevibrations of the armature.

My invention is illustrated in connection with a magnetic soundreproducer of the general type disclosed in the co-pending applicationof Norman C. Pickering, Serial No. 681,800, filed July 8, 1946. Thismagnetic type of sound reproducer comprises a moving system and also afixed portion embodying a generating system or coil. An assembly of thefixed and moving parts of this type of sound reproducer'is shown in thedrawings. Referring to Fig. l, the fixed portion of the sound reproducerassembly comprises a frame made up of a pair of pole pieces 11 ofmagnetic material positioned at the opposite ends of a bar magnet 12.The pole pieces 11 on the opposite sides of the frame 10 projectinwardly toward each other. The pole pieces 11 taper toward their innerends and terminate a short distance from each other leaving a small gapin the magnetic circuit in which the armature 13 is disposed as shown inFigs. 2 and 3. When the unit is assembled, the poles of the magnet 12engaging the pole pieces 11 makes them respectively north and southpoles.

. As seen in Fig. 3, the pole pieces 11 extending inwardly areprovided'with upper and lower fingers. Between these fingers issupported a spool 14 composed of insulating material. Wound on the core16 of the spool 14 is the current generating element in the form of acoil 15. The central opening of the wound coil and the rigid frame 10together with the pole pieces 11.

In order to produce a relatively light but stiff armature, the armature13 is of tubular construction wherein the ratio of stiffness to mass isrelatively high. Thus, it will be seen that the vertical armature 13, asviewed in Figs. 2, 3 and 4, which extends downwardly through the centerof the coil 15 and the gap between the ends of the pole pieces, is oftubular form. Integrally secured to the lower end of the tubular portionis a jewel stylus 18 mounted in the tubular end in a well-known manner.

1 or the other of the pole pieces causing corresponding movement of thearmature 13 around the longitudinal axis of the spring 17. This, inturn, causes a flux in the armature which cuts through the field of thecoil and causes electrical impulses or voltages therein. The coil isconnected to a sound amplifying system through leads 19 and contactprongs 20.

It has been discovered that the operation of a sound reproducer can beimproved by a damping action on the movements of the stylus carryingarmature. It has also been discovered that unctuous substances havingthe viscosity of petrolatum or a viscous lubricating oil having aSaybolt viscosity of over 199 are satisfactory damping media in a soundreproducer of the nature described herein. In other sound reproducers,other unctuous substances with greater viscosity, such as mediumlubricating oil with a Saybolt viscosity of between and 200 may besatisfactory. According to this invention, the damping substanceemployed may be selected in accordance with the characteristics of thesound reproducer parts incorporated in the cast cartridge. A desirabledamping action may also be obtained with certain solid compounds whichhave a stifi viscous action, such as audiod and viscoloid. In thefollowing description, viscosity refers to the property of internalfluid friction in fluids and soft solids which is caused by a slightresistance to. change of form arising from the molecular attractionbetween the moiecules of the substance. The term viscous is used toindicate a substance having a degree of viscosity which will provide asatisfactory damping action to the armature with which it is employed.

The sound reproducer described above is representative of the type ofsound reproducer which may be used in my invention. In this inventionthe sound reproducer is made by casting a resin around the reproduceras- I sembly in a mold. In Fig. l the lower half 21 of a mold and anupper half of the mold ,26. The upper half of the mold 26 is seated onthe lower half 21 and is joined thereto by a dovetail joint 27. Theupper half of the mold is provided with sprues 28 through which thecasting material is introduced into the cavity 25.

In one mode of carrying out this invention, the reproducer parts arefirst assembled. The pole pieces 11 are fastened to their respectiveends of the bar magnet 12 by a suitable adhesive or other means. Thespool 14 with its coil 15 wound thereon is placed between the fingers ofthe pole pieces 11 with the leads 19 extending therefrom. At the end ofthe leads 19, the contact prongs are attached. The armature 13 carryingits stylus 18 is mounted on one end of the spring 17 and the other endof the spring 17 in turn is mounted at the center of the upper surfaceof the bar magnet 12 with the stylus extending downward through the core16 and the coil 15. The assembled reproducer parts are then inserted inthe lower half 21 of the mold opened by the removal of the upper half26. The armature 13 is inserted in the hollow mandrel 24 while the core16 of the spool is slid over the outside of the mandrel 24 so that themandrel 24 fits in the annular area between the armature 13 and the core16. The pole pieces 11 rest on the upper surface of the platform 22. Thepole pieces, thus supported off the floor of the lower half 21 of themold, hold the reproducer assembly in this position while the assemblyis maintained vertical by the spool 14 and armature 13 on the mandrel24. With the assembly thus held in position in the lower half 21 of themold, a viscous grease is applied to the upper side of the reproducerassembly in the vicinity and around the moving parts. As shown in Figs.2, 3 and 4, this viscous grease envelops the free end of the spring 17and armature 13. Additional viscous grease is placed around the armatureby insertion through a lower open end 29 of the hollow mandrel 24. Thegrease inserted through the open end 29 is forced under pressure alongthe surface between the armature 13 and the mandrel 24 until it meetsand merges with the grease placed around the upper end of the armature13. This grease may be inserted in the open end 29 by means of a syringeor any other suitable method. The contact prongs 20 are placed insuitable passages through the wall of the mold at the joint 27 betweenthe upper half 26 and the lower half 21. With the reproducer assemblythus in position, the upper half 26 is placed in position on the lowerhalf 21 and the two halves of the mold are clamped together by suitablemeans.

The surfaces of the cavity are waxed prior to the closing of the mold.The reproducer cartridge of this invention is cast from a resin which ispoured into the cavity 25 through one or more sprues 28. The cavity isvented by similar sprues. The cast resin is allowed to solidify or coolto a hard solid. At the completion of the solidification, the mold isopen and aided by the waxed cavity walls and the grease layer in themandrel 24 and the cast cartridge is easily removed from the mold in theform shown in Figs. 4 and 5.

The cast resin may be any resin which will set and cure from a fluidstate at a temperature in the molding operation which will not cause theviscous grease to melt and run so as to lose its function as a core. Themelt may be either a resin melted to pour into the cavity 25 ordissolved in a suitable solvent to flow into the cavity 25. One suitableresin is a resin or mixture of resins selected from the unsaturatedpolyester resin.

In the case cartridge, the reproducer assembly is encased in a solidresin with a grease core 31 around the armature 13 and the end of thespring 17. Except for the parts encased in the grease core 31, thereproducer parts are firmly embedded in solid resin. To complete theassembly of a cartridge 30, additional viscous grease may be injectedaround the armature 13 in the core 16. Then a small rubber dust shield32 is inserted over the armature 13 and into a recess 33 formed in thebottom of the cast cartridge by the raised platform 22. This dust shieldseals the core 16 around the armature 13. A

ring 34 of a cast phenolic or a metal is snapped in place in the recess33 over the dust shield 32.

The cast cartridge 36 is then complete in the form shown in Figs. 4 and5. The armature 13 is free to move freely at the end of spring 17. Theviscous grease of core 31 serves as a damping medium for the motion ofthe armature and the free end of the spring 17. At the same time, thegrease of the core 31 provides a core around the moving parts of thecast cartridge 30 to allow these parts motion while the remainder of theparts are securely held in the cast resin. In this way the soundreproducer is converted into a single, integral, unitary member and thefreedom of motion of the moving parts is not impaired while the actionof a damping substance is simultaneously incorporated into the castcartridge 30. The viscous grease of the core 31 may be any suitableinert viscous compound of satisfactory consistency. As the castingtemperature is not intended to be greater than F, most heavy petroleumgreases will be satisfactory. In those cases where less damping actionis sought, a lighter or less viscous grease may be employed withcorresponding adjustments in the casting temperature if necessary. Aviscous grease may be prepared from a mixture of silicone oil anddiatomaceous earth to have a consistency of heavy cold cream. Thissilicone diatomaceous earth compound will stand up as a core duringcasting with a resin having a melting point of 150 F. and it will alsohave a viscosity providing desirable damping qualities.

Among the many advantages of this invention are the light weight of thecompleted cast cartridge and the solidity of its unitary construction.Also, the parts of the reproducer assembly are completely protected fromatmospheric action. In sound reproducers any avoidance of resonance orspurious noises caused by loose parts is desirable. Encasing theassembly parts in a solid body to hold them permanently and securelywith relation to each other insures the reduction, if not completeelimination of undesirable interference or operation of the parts.

In addition, the permanent and unvarying damping action of the sealed-ingrease core material is advantageous. Other advantages are the economyof manufacture both in materials and in the assemblying operation. Also,the easy adaptation of the reproducer of this invention into a varietyof reproducer arms is an advantage. It will thus be seen that by meansof this invention an improved reproducer assembly is provided in arelatively simple and inexpensive manner and a novel and advantageousmethod of producing the reproducer is provided.

Modifications may be made in the illustrated and described embodimentsof this invention without departure from the spirit of the invention. Inthe above description, the cast material has been described as a castresin. Specifically, this refers to the polyester synthetic resin. Thisresin is selected for the example for its compatibility with the lightgrease employed as damping compound and core material in the example.The spirit of this invention is the adaptation of the viscous fluidmaterial and the cast material to combine in the method of thisinvention to provide the product of this invention. The polyester resinreferred to in the specific embodiment above melts and solidifies at atemperature which is close to the melting point of the viscous dampingcompound and core material. Other cast materials may likewise beselected for casting within the temperature limitation of the meltingpoint of the viscous compound. These other cast materials may includeboth synthetic and natural plastic materials without departure from thespirit of this invention.

In another possible modification the casting may be carried on at a"lowered temperature which would also lower the melting point of theviscous grease. Thus, a grease with a lower melting point may beemployed as the damping medium without losing its function as a core inthe casting operation.

A feature of this invention is the dielectric properties of the castingmaterial. Likewise, the viscous compound may be varied in physicalproperties according to the degree of damping desired in the castcartridge. In application to this invention where the damping compoundis changed, the cast material may be correspondingly changed insubstance Without departure from this invention. For this reason, thisinvention is limited by the scope of the accompanying claims.

I claim:

1. A method of forming a unitary sound reproducer cartridge having partsfor sound reproduction including moving parts comprising the steps ofmounting the sound reproducing parts in a mold, inserting a hollow shaftof said mold within the coil of said sound reproducing parts, immersingat least part of the moving parts of said sound reproducer in a core ofa grease substance having a Saybolt viscosity of 100 or more, casting aresin in said mold at a temperature below 150 F. substantiallycompletely enclosing said reproducer parts and said core in said onepiece resin casting and removing said casting and said enclosed core andparts from said mold and removing said hollow shaft from said coil andsaid one piece resin casting to provide a small access passage from saidreproducer parts through said casting.

In a magnetic sound reproducer the combination of a core of a greasehaving a Saybolt viscosity of 100 or more, a reproducer assemblyassociated with said core, moving parts of said reproducer assembly atleast partially immersed in said core, a solid one piece resin castingsubstantially completely enclosing said core and said reproducerassembly save for a single narrow passage in said resin castingextending outward from said core and an armature of said reproducerassembly extending out of said casting from said core through saidnarrow passage.

3. In a magnetic sound reproducer a combination of a core of a greasehaving a Saybolt viscosity of at least 100, a reproducer assemblyincluding in combination a pair of pole pieces, a coil, a bar magnet andan armature supported on the free end of a spring mounted on said barmagnet, said armature and said free end of said spring immersed in saidcore; a solid one piece resin casting substantially completely enclosingsaid reproducer assembly and core having said bar magnet, pole piecesand coil embedded therein, a single narrow passage through said resincasting and an end of said armature extending out of said castingthrough said passage.

4. In a magnetic sound reproducer, the combination of a core of viscousfluid having a Saybolt viscosity of at least 100, a solid one-pieceresin casting enclosing said core, a reproducer assembly partiallyimmersed in said viscous fluid and partially embedded in said resincasting including in combination a pair of pole pieces, a coil, a barmagnet and an armature mounted on the free end of a spring mounted onsaid bar magnet, said bar magnet being embeddedin said casting and saidarmature and said free end of sai spring being immersed in said core,and said solid one-piece casting substantially completely encompassingsaid reproducer assembly and said core save for a single, narrow passagethrough said casting and an end of said armature extending out of saidcasting through said passage.

5. A magnetic sound reproducer as claimed in claim 4 having said coiland said pole pieces and terminal leads embedded in said resin castingand contact prongs partially embedded in said resin casting andextending therefrom.

6. A method of forming a unitary sound reproducer cartridge having partsfor sound reproduction including moving parts comprising mounting thesound reproducing parts in a mold, immersing a portion of said movingparts of said sound reproducer in a viscous substance having a Sayboltviscosity of or more, casting a resin in said mold, embedding a portionof said reproducer parts in said resin and enclosing said core and saidmoving parts in said resin casting, providing a small access passagethrough said resin casting, removing said casting and said enclosed coreand parts from said mold to provide said unitary sound reproducer with asubstantially completely enclosing solid one-piece resin casting formedaround said reproducer parts and said core.

References Cited in the file of this patent UNITED STATES PATENTS1,333,004 Vaughn Mar. 9, 1920 2,037,255 Miller Apr. 14, 1936 2,400,662Roberton May 21, 1946 2,471,592 Thomson May 31, 1949 2,519,185 HarrisAug. 15, 1950 2,538,164 Pickering Jan. 16, 1951 2,543,127 Pickering Feb.27, 1951 2,548,353 Cunningham Apr. 10, 1951 2.5 948 Lynch Apr. 29, 1952

